The stimulation of various cells of the body has been used to produce a number of beneficial effects. One method of stimulation involves the use of electrodes to introduce an externally generated signal into cells. One problem faced by electrode-based brain stimulation techniques is the distributed nature of neurons responsible for a given mental process. Conversely, different types of neurons reside close to one another such that only certain cells in a given region of the brain are activated while performing a specific task. Alternatively stated, not only do heterogeneous nerve tracts move in parallel through tight spatial confines, but the cell bodies themselves may exist in mixed, sparsely embedded configurations. This distributed manner of processing seems to defy the best attempts to understand canonical order within the CNS, and makes neuromodulation a difficult therapeutic endeavor. This architecture of the brain is poses a problem for electrode-based stimulation because electrodes are relatively indiscriminate with regards to the underlying physiology of the neurons that they stimulate. Instead, physical proximity of the electrode poles to the neuron is often the single largest determining factor as to which neurons will be stimulated. Accordingly, it is generally not feasible to absolutely restrict stimulation to a single class of neuron using electrodes.
Another issue with the use of electrodes for stimulation is that because electrode placement dictates which neurons will be stimulated, mechanical stability is frequently inadequate, and results in lead migration of the electrodes from the targeted area. Moreover, after a period of time within the body, electrode leads frequently become encapsulated with glial cells, raising the effective electrical resistance of the electrodes, and hence the electrical power delivery required to reach targeted cells. Compensatory increases in voltage, frequency or pulse width, however, may spread of electrical current may increase the unintended stimulation of additional cells.
Another method of stimulus uses photosensitive bio-molecular structures to stimulate target cells in response to light. For instance, light activated proteins can be used to control the flow of ions through cell membranes. By facilitating or inhibiting the flow of positive or negative ions through cell membranes, the cell can be briefly depolarized, depolarized and maintained in that state, or hyperpolarized. Neurons are an example of a type of cell that uses the electrical currents created by depolarization to generate communication signals (i.e., nerve impulses). Other electrically excitable cells include skeletal muscle, cardiac muscle, and endocrine cells. Recently discovered techniques allow for stimulation of cells resulting in the rapid depolarization of cells (e.g., in the millisecond range). Such techniques can be used to control the depolarization of cells such as neurons. Neurons use rapid depolarization to transmit signals throughout the body and for various purposes, such as motor control (e.g., muscle contractions), sensory responses (e.g., touch, hearing, and other senses) and computational functions (e.g., brain functions). Thus, the control of the depolarization of cells can be beneficial for a number of different purposes, including (but not limited to) psychological therapy, muscle control and sensory functions. For further details on specific implementations of photosensitive bio-molecular structures and methods, reference can be made to “Millisecond-Timescale, Genetically Optical Control of Neural Activity”, by Boyden, Edward S. et al., Nature Neuroscience 8, 1263-1268 (2005). This reference discusses use of blue-light-activated ion channel channelrhodopsin-2 (ChR2) to cause calcium (Ca++)-mediated neural depolarization, and is fully incorporated herein be reference. Other applicable light-activated ion channels include halorhodopsin (NpHR), in which amber light affects chloride (Cl−) ion flow so as to hyperpolarize neuronal membrane, and make it resistant to firing.
While these and other methods are promising scientific discoveries, there is need for innovations that allow for practical application of these basic mechanisms, such as in vivo neuromodulation, for example, to treat diseases in humans. Often, the specific location at which the photosensitive bio-molecular structure is applied to is critical. Moreover, the process by which light is made able to reach the photosensitive bio-molecular structures can involves obstacles, on the practical level. In many applications, minimal invasiveness of the procedure is paramount. For instance, the brain is a delicate organ and less disruption is usually a paramount issue for surgeries and similar procedures on the brain. Thus, it is sometimes desirable that the extent of any surgical procedure be kept to a minimum. This can be difficult, however, where large devices are needed for the administration of treatment. In some applications the comfort of the patient is also important. Thus, external apparatus can be less than ideal.
These and other issues have presented challenges to the implementation of the stimulus of target cells, including those involving photosensitive bio-molecular structures and those used in similar applications.